Re: "Charged" capacitor mis-terminology

["Prof. John P. Ertel (wizard)" <jpe@nadn.navy.mil>]

We might want to remember that a number of our students will later be
exposed to some part of the engineering world.  For those folks (as well
as in older physics texts), there is a current between the plates a a
capacitor (even one with "free space" between the planes).  The current of
which they speak is the  DISPLACEMENT CURRENT .  Generally, they say that
the current at any point in space is the sum of the CONDUCTION &
DISPLACEMNET CURRENTS 

	I(total) = I(cond) + I(disp)

It is, of course, a mathematical crutch so that the don't have to amend
the naive form of Kirchhoff's 2nd rule/law.  If we would only teach the
CONTINUITY EQUATION, we would have no need of I(disp).  The sum of the
currents into or out of a junction need not be zero it there is a build-up
of charge at that point.

I maintain that the CONTINUITY EQUATION is not difficult to understand at
all.  Rather, every student I have ever talked to about this has
understood the CONTINUITY EQUATION very well.  OK, maybe they have never
thought of the mathematical statement, but they know and understand it
just the same.

Not convinced?  Just ask them to explain the following situation:

	Suppose that you have a wall in a building into one
	side of which runs a pipe.  On that pipe there is
	a meter that reads 5 gal/min flow.  On the other side
	of the wall, there is another pipe that may even seem
	to be an extension of the pipe on the other side.
	Suppose that the meter on this pipe read 4 gal/min
	flow.

	How come the fluid flow into the wall isn't the
	same as the fluid flow out of the wall on the other
	side?


	Every student that I have ever asked has correctly
	come up with both of the only two possible solutions:

	1) there is at least one other pipe within the wall;
	2) fluid is accumulating within the wall at the rate
	of 1 gal/min.

How could anyone ever think that most students don't know or understand
the CONTINUITY EQUATION.  They know and understand the concept very well
--- the simply don't recognize it in the mathematical form that we
commonly show them.

I usually find that if I take time to teach them only a little extra math
(that they may not already know) I don't have to make up "virtual
currents" to make Kirchhoff's Current Law work.


On Wed, 18 Feb 1998, Mark Shapiro wrote:

> 	Charge never flows through a capacitor (it can flow in the
> external circuit).....
>
> Dr. Mark H. Shapiro
> Physics Department
> California State University, Fullerton
> P.O. Box 6866
> Fullerton, California 92834-6866
>
> Phone: ++ (714) 278-3884	PCS: ++ (714) 350-3575
> Fax:   ++ (714) 278-5810
> e-mail: mshapiro@fullerton.edu

ERTEL SENDS.
                             /^\-/^\
                            /       \
                           |    *    |
                           |#########|
                   ===========================
                     / Prof. John P. Ertel \
                    /  veteran Eagle Scout  \
       +==================================================+
       |   jpe@nadn.navy.mil   or    ertel@nadn.navy.mil  |
       | Physics Department, 9C     Office : 410-293-6657 |
       |   Michelson  Rm-338           DSN :     281-6657 |
       |    572 Holloway RD            FAX : 410-293-3729 |
       |  U. S. Naval Academy     Research : 410-293-2759 |
       | Annapolis, MD 21402-5026     Home : 410-757-6618 |
       +==================================================+